Field of the Invention
The invention relates to a differential protection method for monitoring a line of a power grid in which current indicator measured values are measured in each case with measuring devices at the ends of the line, the values indicating the amplitude and phase angle of a phase current flowing at the respective end of the line, wherein the measuring devices have local timers and allocate a timestamp to the current indicator measured values indicating the time of their measurement, at least the current indicator measured values measured at one end are transmitted via a communication connection to an evaluation device, a differential current value is formed through vectorial addition by means of the evaluation device with current indicator measured values temporally allocated to one another, wherein time delay information indicating the time delay between the local timers of the measuring devices is used for the temporal allocation of the current indicator measured values measured at different ends, and a fault signal indicating a fault affecting the line is generated if the differential current value exceeds a predefined threshold value.
The invention also relates to a corresponding differential protection device to carry out a differential protection method of this type, and a differential protection system with at least two differential protection devices of this type.
A current differential protection method (referred to below for the sake of simplicity as a “differential protection method”) is often used to monitor high-voltage and medium-voltage lines, such as overhead lines or cables, of power grids. The current flowing at the ends of the monitored line is measured in the form of current indicator measured values providing information relating to the amplitude and phase angle of the current and is fed to a differential protection device. The differential protection device forms differential current values from the current indicator measured values through vectorial addition, the values being used to evaluate the operational situation of the monitored primary component. To do this, the differential current values are normally determined through vectorial addition and subsequent amount formation from the current indicator measured values. In the fault-free case, the differential current values are in a range close to zero, since, in simplified terms, the current flowing into the component flows completely out of it again. Conversely, if differential current values arise which exceed a non-zero threshold value, these indicate a fault-affected operating condition, e.g. an internal short circuit. In this case, the fault current that is present must be interrupted by opening switching devices, e.g. power switches, which limit the line. For this purpose, the differential protection device generates a corresponding fault signal, as a result of which the generation of a switching signal for the respective switching device can be instigated.
In the case of a line with ends located far apart, for example a line having a length of several kilometers, the current indicator measured values must be transmitted over a longer route. In such a case, a separate differential protection device is normally disposed at each of the ends of the line, forming the respective differential current value from the respective (locally measured) current indicator measured values and the current indicator measured values received from the other end of the line. In the case of a line with a plurality of ends, e.g. a branched line, current indicator measured values are additionally required from each of the ends in order to be able to carry out the differential protection method correctly. To do this, the current indicator measured values measured locally at the respective measuring points must be transmitted between the individual differential protection devices.
Current indicator measured values from at least two different measuring points at the respective ends of the monitored line are consequently required in order to evaluate the operational situation of the line. In the case of existing differential protection systems, the current indicator measured values are frequently transmitted via a hard-wired point-to-point connection (e.g. copper or optical fiber lines), as a result of which a deterministic transmission is achieved, i.e. the transmission time of the measured values is mainly dependent on the transmission route and the transmission type and is also essentially constant.
In more recent differential systems, a tendency has now developed to transmit the current indicator measured values via a communication network, e.g. a telecommunication network or a data communication network based on the IP protocol, rather than via a hard-wired connection. This offers the advantage of a more economical communication infrastructure. Furthermore, communication networks are often already present in the vicinity of primary electrical components, e.g. between substations of a power grid, and can be used without additional costs for the transmission of the current measured values.
However, the advantage of the deterministic transmission time is often lost through the use of communication networks, so that a problem arises in terms of the allocation of the respectively associated current measured values. The local and the received measured values must in fact be temporally aligned in such a way that the indicator measured values measured at the same time are compared in each case with one another in the differential value formation. If, as is possible in a deterministic communication system, for example, the respective transmission time of the current indicator measured values is known, the respective time of the measured value measurement can be determined from the reception time of the measurement data in the local differential protection device and the known transmission time. Conversely, in communication systems which are not deterministic in terms of the transmission paths and/or transit delays of messages, for example telecommunication networks or IP or Ethernet networks, problems arise, for example, because the transmission time is not constant or differences occur in the transmission time in the forward and backward direction.
An over-function of the differential protection device can be caused by such uncertainties in the temporal alignment of the respective current indicator measured values, since a differential current value which specifies a fault relating to the primary component, but which does not actually exist at all, is formed in the addition with the correct algebraic sign of non-associated indicator measured values. This may result in inadvertent tripping responses which impair the correct operation of the power grid.
Since it is thus primarily important for a reliable mode of operation of a differential protection method that the current indicator measured values on the line ends are determined in each case at the same times, the measuring devices with which the current indicator measured values are measured at the line ends now normally have local timers or clocks which emit a time signal which is used for the time stamping of the measured current indicator measured values. In order to be able, for example, to determine the indicator measured values in each case at the same times, it is necessary to time-synchronize these local timers with one another so that the time delay between the local timers is adjusted to zero. This could be achieved, for example, by synchronizing the local timers with one another via an external timer system, e.g. by the time signal contained in the GPS signal. For this purpose, however, special receiving systems, e.g. GPS receivers, are necessary, which increasingly impact on the price of the device. The antennas of GPS receivers furthermore require an unobstructed line of sight to the satellites, so that corresponding structural conditions must prevail or be created.
Without external means such as GPS receivers, a synchronization of the local timers is frequently carried out today using messages transmitted between the measuring devices at the line ends. With this method, also referred to as the “ping-pong method,” the transmission and reception timestamps of the transmitted messages are exchanged between the respective measuring devices. The time delay between the local timers is obtained as a result of this method. It is thus possible either to convert the time information of the received measured values of the respective other measuring device contained in the timestamp of the message into their own time, or to adjust the time delay to zero by adjusting the time of one of the timers.
However, a fundamental requirement for this method for determining the time delay between the timers is that the transit delays of the messages must be the same in the transmission on the forward path and the return path. In this connection, this is referred to as the guarantee of symmetrical transit delays.
Communication networks normally guarantee a high data quality, availability and symmetrical transit delays. Nevertheless, it may occur that the requirement for symmetrical transit delays in a communication network cannot be completely met. It has been observed, for example, in the case of a communication route of the transmitted messages passing through a plurality of communication networks or a plurality of subnetworks of a communication network, the transit times may increase gradually in very small amounts and may then in turn suddenly decrease in large jumps. It has furthermore been observed that the symmetry of the transit delays changes when the route of the communication route changes and the originally present transit times do not prevail on the return to the original communication route.
If asymmetrical transit times occur, i.e. the transit times of the messages on the forward path and the return path are not the same, the received current indicator measured values cannot be temporally allocated correctly to their own current indicator measured values. A so-called “angular error” occurs. The respective differential protection device would consequently determine a differential current value which is not present in reality in relation to the monitored line. If this differential current value exceeds a specific threshold value, this results in an inadvertent shutdown of the monitored line.
A differential protection method is known from U.S. Pat. No. 8,154,836 B2 in which, in a differential protection system with more than two ends, the local timers are synchronized via an external synchronization method, e.g. a GPS signal, or a line-based synchronization method. A combined use of the different methods is also possible. If an external time signal fails, it is possible to switch over to the line-based synchronization method under certain conditions so that the timers can continue to be kept synchronous.